Difference between revisions of "FAQ for AMS Data Users"
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== How are the detection limits estimated? == | == How are the detection limits estimated? == | ||
| + | |||
| + | Detection limits are typically estimated from measurements with the normal AMS modes while sampling through a total particle filter. Typical detection limits are given in Table XX of DeCarlo et al. (2006). | ||
== Do the detection limits change with the averaging time? == | == Do the detection limits change with the averaging time? == | ||
| − | + | Yes, detection limits change a lot with the averaging time used, and they are not meaningful unless the averaging time is also reported. From previous results, the DLs scale as the inverse sqrt of the averaging time. | |
| − | + | DL estimates were derived | |
with the method of Drewnick et al. (Atmos. Meas. Tech., 2, 33-46, 2009). | with the method of Drewnick et al. (Atmos. Meas. Tech., 2, 33-46, 2009). | ||
| Line 28: | Line 30: | ||
organic species. | organic species. | ||
| + | |||
| + | == What is the diameter measured by the AMS? == | ||
| + | |||
| + | These are vacuum aerodynamic diameters (d_va, see DeCarlo et al. Aerosol Sci. Technol. 38, 1185-1205, 2004), which are proportional to particle density, and decrease with increasing shape factor for non-spherical particles. | ||
| + | |||
| + | == What is the size cut of AMS measurements? == | ||
| + | |||
| + | Sampling was through the Clarke aircraft inlet, which has an upper size cut of several microns. The actual size cut was limited by the AMS, and is estimated as 35 nm for zero tranmission at the low end (Zhang et al., Environ. Sci. Technol. 38, 4797-4809, 2004). For this PCI/inlet setup, the 50% transmission at the upper end was determined at around X nm. | ||
| + | |||
| + | == What are the accuracy and precision of AMS concentration measurements? == | ||
| + | |||
| + | Accuracy estimate (2sdev): Inorganics 34%, Organics 38%, dominated by uncertainity in the particle collection efficiency due to particle bounce (Eb, see Huffman et al., Aerosol Sci Technol. 39, 1143-1163, 2005). Precision at low concentrations is the same as the detection limits reported below. Precision at higher concentrations is much better than the accuracy. | ||
Revision as of 15:56, 29 June 2009
The purpose of this page is to serve as a repository of answers to frequently asked questions from users of AMS data products. Contributions are most welcome, and should be directed to the curator of the page (JL Jimenez as of June 2009).
Contents
- 1 What are ug sm-3?
- 2 How are the detection limits estimated?
- 3 Do the detection limits change with the averaging time?
- 4 Is the AMS "organics" the same as "OC" or "organic carbon" measured with other instruments and reported by some models?
- 5 What is the diameter measured by the AMS?
- 6 What is the size cut of AMS measurements?
- 7 What are the accuracy and precision of AMS concentration measurements?
What are ug sm-3?
ug sm-3 are ug m-3 converted to standard pressure and temperature (STP) conditions of 1013 mbar, 273K without any evaporation/condensation. These units are very useful when comparing data taken over a range of pressure and temperatures, such as aircraft data. They are also very useful when comparing aircraft data to data from ground sites, or data from different ground sites which have different ambient pressures and/or temperatures.
Ratios to gas-phase species (e.g. OOA/Ox) are constant when expressed in ug sm-3 ppb-1, but not if ug m-3 are used.
Note that unfortunately there are many other definitions of STP, many of which are compiled in this Wikipedia article. Some of them are also used in the aerosol field, so be aware of these possible differences.
All papers reporting ug m-3 of aerosol should state the basis of the m-3 in the experimental section. We recommend the use of the unit ug sm-3 for concentrations reported under STP conditions, and of ug am-3 for concentrations reported under ambient conditions.
How are the detection limits estimated?
Detection limits are typically estimated from measurements with the normal AMS modes while sampling through a total particle filter. Typical detection limits are given in Table XX of DeCarlo et al. (2006).
Do the detection limits change with the averaging time?
Yes, detection limits change a lot with the averaging time used, and they are not meaningful unless the averaging time is also reported. From previous results, the DLs scale as the inverse sqrt of the averaging time.
DL estimates were derived
with the method of Drewnick et al. (Atmos. Meas. Tech., 2, 33-46, 2009).
Is the AMS "organics" the same as "OC" or "organic carbon" measured with other instruments and reported by some models?
"Organics" correspond to organic
aerosol mass (often referred to as OM or OA). It includes OC (organic carbon), as well other atoms (H,O,N) in
organic species.
What is the diameter measured by the AMS?
These are vacuum aerodynamic diameters (d_va, see DeCarlo et al. Aerosol Sci. Technol. 38, 1185-1205, 2004), which are proportional to particle density, and decrease with increasing shape factor for non-spherical particles.
What is the size cut of AMS measurements?
Sampling was through the Clarke aircraft inlet, which has an upper size cut of several microns. The actual size cut was limited by the AMS, and is estimated as 35 nm for zero tranmission at the low end (Zhang et al., Environ. Sci. Technol. 38, 4797-4809, 2004). For this PCI/inlet setup, the 50% transmission at the upper end was determined at around X nm.
What are the accuracy and precision of AMS concentration measurements?
Accuracy estimate (2sdev): Inorganics 34%, Organics 38%, dominated by uncertainity in the particle collection efficiency due to particle bounce (Eb, see Huffman et al., Aerosol Sci Technol. 39, 1143-1163, 2005). Precision at low concentrations is the same as the detection limits reported below. Precision at higher concentrations is much better than the accuracy.
